The present invention relates to the configuration of a thin-film transistor used in an electro-optical device such as a liquid crystal display device.
In an electro-optical device that is represented by an active matrix type liquid crystal display device, there is known a configuration in which thin-film transistors (TFTs) are used as drive elements and switching elements. Thin-film transistors are configured using a semiconductor (usually, silicon semiconductor) thin film that is formed on a glass substrate by vapor-phase deposition. Thin-film transistors are also used for an image sensor and other devices.
FIGS. 4(A) and 4(B) are a sectional view and a top view, respectively, of a one-pixel portion of an active matrix circuit using conventional thin-film transistors. FIG. 4(A) is taken along line A-Axe2x80x2 in FIG. 4(B). A thin-film transistor whose cross-section is shown in FIG. 4(A) is composed of a glass substrate 401, an amorphous silicon or crystalline silicon semiconductor active layer formed on the glass substrate 401 and having a source region 402, a channel forming region 403 and a drain region 404, a gate insulating film 405 made of silicon dioxide or silicon nitride, an interlayer film 407 made of silicon dioxide, a drain contact portion 412, a source contact portion 411, a drain electrode 410, and a transparent conductive film (ITO film or the like) 408 that is connected to the drain electrode 410 and constitutes a pixel electrode. (The source and drain are reversed for a certain operation of the TFT.)
The source region 402 of the thin-film transistor shown in FIG. 4(A) is connected to a source line 409 via the source contact portion 411. A gate electrode 406 is connected to a gate line 413. While usually the source line 409 and the gate line 413 are perpendicular to each other, this is not always the case. In general, the gate electrode 406 and the gate line 413 are made of a metal such as aluminum or a semiconductor such as polysilicon added with phosphorus.
FIGS. 4(A) and 4(B) show a single pixel. Actually, an active matrix circuit is formed by disposing at least one pixel as shown in FIGS. 4(A) and 4(B) at each intersection of the source lines and the gate lines. A liquid crystal panel is configured by sealing a liquid crystal material between an active matrix substrate on which the active matrix circuit is formed and an opposed substrate. There are following types of liquid crystal display devices that use the liquid crystal panel having the above configuration.
(1) Liquid crystal display is realized by applying light (back light) to the liquid crystal panel.
(2) A video image is produced by applying high-intensity light to the liquid crystal display panel and projecting, onto a screen, light that is transmitted from the liquid crystal panel. (Liquid crystal projector)
(3) A reflecting plate is disposed on the back side of the liquid crystal display panel, and display is effected by causing external light to be reflected by the reflecting plate.
In particular, where light is irradiated from the glass substrate side in cases (1) and (2) above, it is necessary to shield the active layer, particularly the channel forming region, from the illumination light. This originates from the fact that the active layer (semiconductor layer denoted by 402-404 in FIGS. 4(A) and 4(B)) is made of amorphous silicon or crystalline silicon such as polysilicon. In general, the resistance of a silicon semiconductor varies when it receives light. In particular, when an amorphous silicon or crystalline silicon film is used, which includes dangling bonds, its electrical characteristics are greatly varied by illumination with high-intensity light. Further, the resistance of an intrinsic semiconductor (used for the channel forming region) is varied more greatly by illumination with light than an N-type or P-type semiconductor (used for the source and drain regions). Therefore, it is absolutely necessary to prevent the channel forming region from being illuminated.
Where light 414 is incident from above the gate electrode 406 in the thin-film transistor (top gate TFT) having the structure in which the gate electrode 406 is located over the semiconductor active layer (see FIGS. 4(A)), it seems that no light may enters the channel forming region 403 due to the gate electrode 406 serving as a mask.
Actually, however, part of the incident light goes around the gate electrode 406, to enter the channel forming region 403. As a result, the conductivity of the channel forming region 403 is varied by the illumination and its characteristics are also varied.
That is, it is impossible for only the gate electrode 406 to completely prevent light from entering the channel forming region 403. This problem is remarkable when the channel forming region and the gate electrodes are formed in a self-aligned manner. To solve this problem, a light shielding layer or film is effectively used, but this increases the number of manufacturing steps.
An object of the present invention is to realize, without increasing the number of manufacturing steps, a structure of a thin-film transistor which is basically of the type shown in FIGS. 4(A) and 4(B) and in which light is not incident on nor enters the active layer, particularly the channel forming region. In particular, the invention is effective when applied to an active matrix circuit having thin-film transistors whose sources and drains are formed by a self-align process that produces a very small overlap of the gate electrode and the source and drain regions.
The invention is characterized in that in an active matrix circuit, a source line or an electrode or wiring line (they cannot be clearly discriminated from each other) extending from the source line is so formed as to cover a channel forming region, to use the source line or an electrode or wiring line as a light shielding layer for the channel forming region. Since the channel forming region is included in a portion where a gate electrode and a semiconductor active layer overlap with each other, forming a source line or an electrode or wiring line extending from the source line to cover such a portion is approximately equivalent to the above.
In the above structure, examples of a substrate having an insulative surface are a glass substrate, a plastic substrate, a metal or semiconductor substrate on whose surface an insulating film is formed.
FIG. 2(D) shows an example of the structure in which the source line or the electrode or wiring line extending from the source line of the thin-film transistor is so formed as to cover at least the channel forming region. In FIG. 2(D), a source line electrode/wiring line 112 that is connected to a source region 104 of a thin-film transistor via a contact portion 108 covers a channel forming region 105. That is, in FIG. 2(D), both of the electrode/wiring line 112 and the gate electrode 107 serve as light shielding films for the underlying channel forming region 105.
It may be possible to realize a similar structure by using an electrode/wiring line for connecting the pixel electrode 103 and the drain region 106. However, in such a case, a parasitic capacitance between the pixel electrode 103 and the gate electrode 107 becomes large and a potential variation of the gate line 107 affects the pixel electrode 107, causing serious problems in the operation of the active matrix. (For example, refer to H. Ono Kikuo et al: xe2x80x9cFlat-Panel Display ""91,xe2x80x9d page 109.)
On the other hand, in the invention, a capacitive coupling between the source line and the gate line due to a parasitic capacitance adversely affects the operation speed of the thin-film transistor, which does not influence the pixel potential. Therefore, no problem occurs in terms of the image display. Further, in the invention, a parasitic capacitance that occurs due to the source line laid over the gate electrode does not cause any substantial problem, because it is smaller than {fraction (1/10)} of the parasitic capacitance at the intersection of the source line and the gate line of the active matrix.
FIG. 1(A) is a specific top view of the structure whose cross-section is shown in FIG. 2(D). FIG. 1(B) is a circuit diagram corresponding to FIG. 1(A). As is apparent from FIG. 1(A), although the pixel electrode 103 is connected to the drain region 106 via a contact portion 109 and a wiring line, the wiring line does not cover the channel forming region. On the other hand, an extension 112 of the source line, which is connected to the source region 104 of the thin-film transistor via a contact portion 108 is so formed as to cover the channel forming region (i.e., the portion where the gate electrode 107 and the semiconductor active layer overlap with each other), and serves as a light shielding layer.
In summary, the problem that the characteristics of a thin-film transistor is varied or deteriorated due to illumination light coming from the electrode side can be solved by constructing the electrode/wiring line that is connected to the source (or drain) of the thin-film transistor so that it serves as a light shielding layer for the channel forming region. The electrode/wiring line that is connected to the drain (or source) is connected to the pixel electrode.
To construct a liquid crystal display panel by using the invention, it is necessary that an active matrix substrate be illuminated from above, i.e., that a light source, an opposed substrate, and the active matrix substrate be arranged in this order. The light shielding effect of the invention is entirely lost if the active matrix substrate is illuminated from below, i.e., from the back side of the thin-film transistor.